This invention pertains to adhesives useful in the lamination of lignocellulosic materials such as wood, especially in plywood manufacture for the bonding of thin overlays or veneers to softwood cores. Adhesives for plywood preparation, for example, must have certain working characteristics in addition to the proper adhesive properties for satisfactory performance under modern production methods. Even though good strengths may be possible under certain conditions with particular resins, the resins may not be suitable for plywood adhesives if the working characteristics are such that these results cannot be readily achieved under the accepted production methods employed in plywood manufacture.
In many processes, the adhesive is applied by spraying. Thus, in addition to the requirements that the adhesive be fast setting, have a good pot life, and have the required penetration to form a strong bond, the consistency of the adhesive has to be such that it can be readily sprayable.
Further, in many operations the assembled panels of plywood or laminated wood products are pre-pressed cold prior to heat setting of the adhesive by hot pressing. By pre-pressing the assembled panels, the capacity of the heated platen presses is increased and the quality of the laminated wood products or plywood obtained can be materially improved. In cold pre-pressing, the adhesive must form a sufficient bond to permit the handling of the pre-pressed panels without shifting or separation of the plies after the pressure is removed. The laminate or consolidated panel may be stored for various lengths of time ranging from a few minutes to hours after pre-pressing before the panels are subjected to the high temperature and pressure to finally set the adhesive. Thus the adhesive, in addition to providing the necessary initial bond, must remain after pre-pressing in a form capable of forming a strong final bond upon heat setting.
Adhesives containing urea-formaldehyde resins which are light in color are often used in overlay plywood manufacture. However, the characteristics of the adhesives are such that the adhesives may not be applied by spraying. Phenol-formaldehyde adhesives often used in plywood operations and wood laminate production are likewise not entirely satisfactory for the overlay production. In the overlay production, the overlay panel or top ply applied to the cores is commonly less than 1/16 inch in thickness, generally being 1/20 to 1/30 of an inch in thickness. It is usually of hardwood or a wood of light color. Due to the thinness and porosity of the top ply, the urea-formaldehyde resin adhesives will "bleed through" the veneer imparting dark streaks in the finished panel.
Heretofore it has been found desirable to employ gluten-containing vegetable or cereal grain flours as extenders for condensation-type resin adhesives. Such extenders are used primarily to reduce costs. An aqueous medium is generally a necessary ingredient when a gluten-containing extender is used in combination with a condensation-type resin adhesive as, for example, when the adhesive is spread over a surface to be glued or assembled. When such gluten-containing vegetable or grain flours are used as extenders, the amount of water required for the mix depends primarily upon the gluten content of the particular flour used.
In such practice, however, serious problems have arisen because, when such gluten-containing extenders have been used in substantial amounts, the "free flow" of the resin composition is retarded and the liquid adhesive is thus found to be thicker and spreadable only with difficulty. Such compositions become stringy and have a porridge-like consistency when dissolved or dispersed in water. In addition, such resin adhesives, when highly extended with a vegetable or grain flour, inevitably require the addition of large quantities of water to enable them to become spreadable. This raises further problems, however, in that when such large amounts of water are used various difficulties arise in the subsequent processing operations as, for example, when these adhesives are employed in wood laminating in hot-pressing operations. The large amount of water employed is converted into steam during the hot-pressing operations which in turn causes the formation of blisters between the laminae, thus making the finished article unfit for commercial use.
Wheat flour is used in the art of adhesives as an extender; in urea-formaldehyde hardwood plywood glues it can represent, for example, 25 percent by weight of the total glue. In phenol-formaldehyde plywood glues, the wheat flour can vary from 2.5 percent to 7 percent of the total glue and up to 54 percent of the total filler plus extender, with wide variations depending on the bran content of the flour. Since wheat flour is a staple foodstuff, it is a relatively expensive ingredient for glue mixes, and its availability varies with the world commodities market. A less expensive substitute for wheat flour which will perform at least as well as wheat flour in such glue mixes is therefore desirable for plywood manufacturing and other adhesives applications.
For example, U.S. Pat. No. 4,070,314, issued Jan. 24, 1978 to Alexander et al., discloses the use of high fiber-containing fractions from cereal grains or oil seeds, optionally in admixture with cereal flours or starches, as amylaceous extenders in synthetic resin-based plywood adhesives. The starches in extenders used in phenol-formaldehyde glues are described as gelatinizing under the highly alkaline conditions prevailing in these phenolic resin systems, thus providing the glue with increased viscosity and tack. However, with urea-formaldehyde glues, most amylaceous (starchy) materials are said to be not satisfactory, since the starch does not become gelatinized in the neutral or slightly acidic environment of the urea-formaldehyde resins. Wheat flours are stated to be the only starch-based extenders employed in contemporary plywood glues, since these materials can increase the viscosity of certain adhesive systems without the starch becoming gelatinized. This property is ascribed to the wheat gluten, which has different properties from the protein portions of other cereal grains. The Alexander patent discloses that certain by-products of the grain milling and processing industries which are high in fiber but relatively low in starch can function effectively as extenders in urea-formaldehyde resin adhesive compositions. These extender materials increase the viscosity of the adhesive compositions, apparently because of water absorption capacities which are higher than those of cereal flours and starches, and are related to the content of protein and pentosans in the fibrous products. The viscosity-increasing fibrous extenders can thus be blended with conventional starchy extenders to produce the desired viscosity in the composition, according to the patent.
U.S. Pat. 4,587,332, issued May 6, 1986 to Lane et al., relates to the production of a viscosity-reduced starch useful in Stein-Hall corrugating adhesives, wherein wheat starch is preferred. (Stein-Hall adhesives contain mainly raw starch, with gelatinized starch as a carrier, sometimes also containing a synthetic resin to improve wet strength bond characteristics.) The starches found useful are described as small granule "B" wheat starches, the crude, lighter weight starch fractions containing the smaller-size starch granules obtained by differential centrifugal sedimentation of the bimodally size-distributed starch granules found in regular or native wheat starches. Starches derived from legumes, e.g., pea starch, or from other cereals such as rye or barley, are also disclosed as useful. The starch fractions found suitable are described as containing indigenous, non-protein, non-starch materials having a hydrophilic colloidal character, such as the wheat pentosans. (The pentosans are complex carbohydrates or hemicelluloses which can be hydrolyzed to yield pentoses.) By increasing the proportion of such materials in modified starches, the lyocratic character (i.e., ability to form colloidal suspensions) of aqueous dispersions of such modified starches are increased. Starches can be treated by various centrifugal processes known in the art to increase the proportions of such components.
Air classification techniques can also be employed to produce such starch fractions; when used to fractionate pea flour, for example, such processes are described as removing large granule starch and protein components, leaving the remaining starch with an increased proportion of non-proteinaceous, non-starch hydrophilic colloids. Such "air classified" legume starches are said to be suitable for use in starch-based corrugating adhesives, except that the raw materials are not available in sufficient quantity. However, Lane et al. is not directed toward the preparation of thermosetting adhesives containing aldehyde condensation resins, such as urea-formaldehyde or phenol-formaldehyde type resinous adhesives.
U.S. Pat. 1,245,980, issued Nov. 6, 1917 to Satow, discloses the use of vegetable proteids obtained from raw materials such as bean, pea, wheat, corn or other leguminous, cereal or grain products, in the production of an electrical insulating compound. Vegetable proteids are extracted and mixed with a glutinizing agent--an aromatic compound such as a phenol. The resulting glutinized mass is treated with a condensing agent such as an active methylene compound, e.g., formaldehyde. In contrast to the normal production of water in a phenol-formaldehyde condensation reaction, the patent states that no condensation of phenol and formaldehyde molecules to form water takes place.
Many other patents relate to the production of adhesive compounds utilizing condensation resins such as urea-formaldehyde or phenol-formaldehyde resin systems in conjunction with starches derived from corn, wheat, potato, tapioca, waxy maize, sorghum, oats, barley, rice, soybeans, seeds, and nuts. See, for example, U.S. Pats. Nos. 2,407,225; 2,580,890; 3,701,743; 3,984,275, and 4,018,959. In addition to the cereal grain and vegetable starches, cellulosic materials useful as adhesive fillers and/or extenders can be obtained from nut shells, wood, barks, leaves, corncobs, coffee bean shells and the like; see U.S. Pats. Nos. 4,244,846; 4,311,621, and 4,562,218. However, the applicant knows of no published art teaching or claiming the use of legume flours such as pea flour as extenders or fillers in curable aldehyde condensation resin adhesives such as urea-formaldehyde or phenol-formaldehyde adhesive compositions.
It was, therefore, a general object of this invention to overcome the disadvantages heretofore encountered, and to improve the spreading and adhesive properties and characteristics of condensation-type resin adhesive compositions employing starchy extenders. Another object of this invention is to reduce the cost of the adhesive without sacrificing desirable properties.
Another object of this invention was to reduce the amount of water required to produce an adhesive composition of a resin and extender combination.
Still another object of this invention was to produce an adhesive composition which would have desirable cure properties, provide improved strength, would not readily blister and which would have other desirable properties when used in plywood hot-pressing operations.